The present invention relates to surface mount technology or design and, more particularly, to systems and methods for assembly of printed circuit boards (PCBs) that employ surface mount technology.
A printed circuit board (PCB) or printed wiring board (PWB) is a composite of materials that provides a surface or substrate on which electronic components or modules may be mounted. PCBs are included in almost all electronic devices including, for example, cellular telephones, television sets, calculators, personal computers, personal digital assistants, telecommunication switches, cameras, etc. The PCB usually provides mechanical support and allows for electrical interconnection among the electronic components or modules.
In general, a PCB typically consists of etched conductors attached to a sheet of insulator. The insulator is generally referred to as the substrate. The substrate is typically made of epoxy-glass-based laminates. The thickness of the laminates may range from less than 0.1 millimeter (mm) to several millimeters. The etched conductors are called traces or tracks. These tracks usually provide electrical interconnections and traverse the outer surface of the board, but may also be formed as inner layers inside the substrate of the substrate of the multilayer board. The electronic components or modules may be mounted on a single side of a board, as well both sides of the board and are typically connected to the conductive tracks by soldering.
The conductive tracks or patterns may be formed by one or more layers of conductors that are interconnected by either through, blind or buried holes or vias, separated by an insulating material, such as epoxy-glass. The conductive pattern typically includes circuit pads (generally referred to as pads throughout) or lands that are used for the connection of the components or modules to the board. A pad may contain a hole or a via. Vias, however, are not usually intended to serve as receptacles for component leads, but instead are generally used as electrical interconnect points or serve to thermally join different conductive layers of a multi-layer structure. The components are usually soldered to the board at the pads either by manual soldering or, more typically, automated soldering, e.g., solder reflow, in a surface mount technology (SMT) or surface mount design (SMD).
SMD typically allows production of more reliable assemblies with higher input/output, increased board density, and reduced weight, volume, and cost. In SMD, the components are assembled or affixed onto the surface of the board without the insertion of the component or module leads into the holes or vias on the board. The components are usually held in place by a sticky mixture of flux. Tiny solder particles are usually applied to the solder pads on the board. The board is conveyed through a oven where the solder paste melts and bonds the components to board and the tracks.
SMD has replaced, to a large extent, conventional through-hole technology, in which the components had leads that passed through holes in the board and which were soldered on the opposite side of the board. As discussed above, in SMD, vias typically serve as interconnection points between the electronic components and the conductive tracks and are usually produced with a small diameter. When a via is formed at the edge of the substrate it typically comprises a semi-circular hole that is called a half via. Half vias are usually formed on most boards during the manufacture. For example, smaller printed circuit boards are usually made from bigger panels by separating the panels using a high speed router, e.g., operating at 50,000 r.p.m., or via a laser cutting operation. After separating the larger panel, half vias are typically formed from the bottom to the top around the edge of the substrate or carrier material and include a metal layer of material (metallization), e.g., gold plated, to form a metallized pad.
Half vias are generally considered problematic. For one, half vias are not as mechanically stable as an undisturbed through via. Furthermore, it is usually challenging to reliably make an electrical connection between a half via in a pad on a module and, for example, a board or substrate on which the module may be mounted. A typical solution to this problem is commonly referred to as plating. In plating, the surface area immediately around a via or hole on the substrate is coated with a metal, which is typically electrolytically or chemically deposited from a bath. Even where plating is employed it is still often difficult to tell whether a solder connection at a half via will provide sufficient electrical connectivity.
Of utility then are processes and systems that will improve the mechanical stability and electrical connectivity between components mounted on a board as part of a SMD.